# Physics - Verification

posted by .

Hello, just wanted to verify is what I did is good. I posted this question yesterday and Bob helped me, so just wanted to be sure it's good.

The problem is:

A bloc of 10 kg is put at the top of an inclined plan of 45 degrees (to the left), attached to a spring which has a spring constant of 250 N/m. The coefficient of kinetic friction between the bloc and the surface of the inclined plan is of 0,300.

What is the maximum elongation of the spring?

First I went to find fc :

EFy = 0
n - mg*sinTHETA
n = mg*sinTHETA*mu
n = 10kg*9.8N/kg*sin45*0.300
n = 20.8 N

then I know that Us = 1/2kx^2
and that Ug = mgycosTHETA*x

so:

Ef = Kf + Ugf

Ef = 1/2kx^2 + mg*cosTHETA*x

Ef = 125x^2 + 29.4x

Ef = Ei + Wnc <-- My friction force

so

125x^2 - 29.4x = 0 - 20.8x
125 x^2 = 8.6x
125x = 8.6
x = 0.0688 meters or 6.8 cm.

does that make sense?

Thank you!

• Physics - Verification -

made a mistake when I wrote Ef = Kf + Ugf

Ef = Kf + Ugf + Usf

• Physics - Verification -

where Kf is = 0

• Physics - Verification -

No it does not make sense.

Workdone by gravity= force down plane*distance= mg*sinTheta*x

Work done on spring: 1/2 k x^2

Work done on friction: mg*cosTheta*mu*x

so
mgSinTheta*x=1/2 kx^2+mgCosTheta*mu*x

divide by x, then group terms

1/2 kx=mg(SinTheta-mu*CosTheta)

125x=98(.7-.3*.7)

x=98/125 *.7*.7= .392m

check my thinking.

• Physics - Verification -

ohhh I see now, but when you say Work done on spring, you take the Energy formula which is 1/2 k x^2 , wouldn't the work on a spring be mu*N*x?

• Physics - Verification -

and why isn't it Wg = Wspring - Wfriction

as friction is going to other way?

• Physics - Verification -

nevermind my first question, my mistake hehe

• Physics - Verification -

your second quesion. Friction absorbs energy, as does the spring. They add to equal the energy input of the system.

• Physics - Verification -

okay cool, thanks a lot Bob

## Similar Questions

1. ### grammar

Ms.writeacher, I'm sorry i was just trying to ask to verify in the grammar if that was what you were referring to. My apologies to you. thank you for your responce . No problem. I just wanted to make sure you found my other responses. …
2. ### physics-bobpursley/drwls

Good morning! I just wanted to say thank you for all of your help. This was my first time taking Physics ever and it was like a foreign language to me. For a long time during this semester I had an "F" average, because of you both …
3. ### Jiskha

Good evening everyone, I just wanted to know why people on this computer keep getting blocked from this site...I tried to ask and answer questions a couple of days ago and was unable to because I was obviously blocked. Whenever i pressed …
4. ### college courses

Good Morning All! so yesterday i posted something about college that i wanted to know. well, i'm at Georgia State right now. i said i wanted to do something in the designing field. i think that's fine because they have a college of …
5. ### science

I was just wondering! I posted a question for you all at 12:01 today. Are you allowed to help me with this type of question?
6. ### Just Curious

Hey. I was just wondering if anybody on here has been to teenink. I can't post the website but if you Google teen ink, it takes you to it. It is a very interesting site for teenage writers. I just wanted to let people who might be …
7. ### Calculus - Verification

Hello, I just wanted to verify this definite integral and if my answer is good: a = 1 and b = 4 integral of (1 - 1/x^2)(x^2 + 1/x) dx = (x + 1/x)(x^3/3 + lnx) + C (4 + 1/4)(64/3 + ln4) - (1 + 1)(1/3 + ln1) = 96.56 - 0.6666 = 95.89 …